714 
DK. WYVILLE THOMSON ON HOLTENIA. 
the stem, have been found in almost all the soundings from the deep sea, indicating 
a multitude of yet to be discovered species of vitreous sponges. 
Our knowledge is not as yet sufficiently extensive to enable us to form a definite 
opinion as to the affinities and sequence of the genera of this order. Holtenia is 
undoubtedly very closely allied in general structure and in the form of its spicules to 
Hyalonema, the principal differences between them being in the general form of the 
sponge, and in the character and arrangement of the long fibres of the “ root.” 
Nor is it yet possible to define with anything like certainty the position of the order 
in the series of the Porifera. There is something in the wonderful complexity of 
design and profusion of ornament in the siliceous skeleton which recalls the Padiolaria, 
and in some cases even the special forms of the spicules are repeated in the two groups. 
(Compare pi. 12, fig. 1, pi. 17, fig. 4, pi. 18, fig. 15, pi. 21, fig. 7, pi. 32, figs. 10, 11, 
pi. 33, figs. 6, 7, &c. of Haeckel’s 4 Die Radiolarien.’) The sarcode is certainly more 
mobile, less loaded with granular matter, and more transparent than in most of the 
other groups of sponges. For sarcode in this condition one would be inclined to anti- 
cipate a somewhat higher form of vital activity. Under an impression that it is through 
this order that the sponges pass into the Padiolaria, I have placed the Porifera Vitrea 
at the head of the series of siliceous sponges, beginning with those genera in which the 
siliceous structures are most varied, and the sarcode least consistent. The typical 
vitreous sponges appear to approach the Padiantia through such forms as Tisiphonia 
and Stelletta , and the Haliciiondrida, probably through Esperia. 
I believe, however, that it is not to the well-known orders of recent sponges inhabiting 
shallow water that the vitreous sponges are most nearly allied. A very remarkable group 
of fossils, the Ventriculites, are highly characteristic of the cretaceous formation. These 
are tubular or vase-shaped bodies, presenting a great diversity of graceful form and elegant 
sculpture. The late Mr. Toulmin Smith devoted much time and skill to their investi- 
gation, and came to the conclusion that they were Polyzoa. This view of their position, 
however, presents many difficulties, and the study of Mr. Toulmin Smith’s careful 
drawings and descriptions is in itself sufficient to suggest other affinities. 
The general resemblance between some of the Ventriculites and Euplectella is very 
striking, while others approach the less regular form of Aphrocallistes , and this resem- 
blance extends even to the details of the structure of the network of the wall. The 
striking difficulty is, that while the skeletons of the vitreous sponges to which the com- 
plicated network of delicate tubes forming the wall of the Ventriculites must be com- 
pared, are siliceous; as a rule no silica enters into the structure of the Ventriculites. 
The tubes are empty or filled with loose particles of carbonate of lime, coloured by per- 
oxide of iron. Recent observations have shown that in very many particulars the 
modern deposits in the “warm area” of the Atlantic resemble most closely the deposits 
of the cretaceous seas ; and, as has already been stated by Dr. Carpenter, many animal 
forms identical with chalk fossils, or so nearly allied to them as to justify the assumption 
of modification by variation, or of direct descent, are now being imbedded in the Atlantic 
